The first is that as the neutrino trajectory is bent gravitationally
by density fluctuations in the universe, the deflection of its
spin vector lags behind the deflection of its momentum vector. Thus
a purely left handed neutrino develops an amplitude to be right handed,
and right handed antineutrinos to be left handed; gravitational
fields do not conserve the helicity of neutrinos [1]}, [2]}, [3]}, [4]}.
